1. Field of the Invention
The present invention relates to optical inspection systems, and in particular, to optical systems for inspecting repeating patterns.
2. The Related Art
In the manufacture of integrated circuits (ICs), it is well known to apply a pattern to a substrate by masking a portion of the substrate corresponding to a pattern, and by selectively depositing or removing material on the substrate to form the lines and devices of the IC. Variations in the pattern, including both pattern width and pattern depth, may occur for a variety of reasons. For example, they may occur due to variations in the application of masking material, exposure and development. To achieve the desired accuracy in the ICs, the patterns are often checked during the processing (for end point detection), as well as after completion of circuit processing.
The determination of the shape, depth and width of sub-micron structures on semiconductor wafers can be a difficult task. With sub-micron sized patterns, it has become a common practice to destructively test samples using a scanning electron microscope. This is a time consuming process. It is often desirable to test whether a structure has been formed properly by comparison to a known structure or measurement signature. Such a comparison is useful as an end point detection mechanism, or as an indication of the success of the formation process.
U.S. Pat. No. 4,303,341 to Kleinknecht et al. discusses an inspection method in which a diffraction grating is applied to the substrate being tested. The diffraction grating includes a pattern of evenly spaced lines having a pitch of about 20 micrometers and a line width of 5 micrometers. The substrate is illuminated by a beam of light to form a reflected beam and to form first and second order diffracted beams, having respective intensities I.sub.1 and I.sub.2. The ratio of I.sub.2 /I.sub.1 is calculated. The width of the lines is then calculated from the ratio of I.sub.2 /I.sub.1.
Another method for inspecting repeating patterns has been proposed in Giapis, K. et al., "Use of Light Scattering in Characterizing Reactively Ion Etched Profiles," J. Vac. Sci. Technol. A, Vol. 9, No.3, May 1991, involves the measurement of scattered light intensity. A repeating pattern is illuminated with a laser beam to produce a plurality of diffraction orders. The repeating pattern is spaced widely, with a pitch of approximately 32 micrometers. The large pitch ensures that there are many diffraction orders available for evaluation. The intensity of each diffraction order is then measured. Deviations in the intensity profile are an indication of changes in the repetitive structure.
A disadvantage of the method described above is that, in order to achieve closely spaced diffraction orders, the repeating structures are placed relatively far apart, i.e., 32 micrometers. The patterns on the integrated circuits that are being inspected, however, are closely spaced. This means that the repeating circuit patterns themselves produce very few diffraction orders (when visible wavelengths are used). To overcome this limitation, the inspection method described by Giapis et al. has been performed on test patterns in the form of diffraction gratings on the substrate, instead of using the actual circuit patterns themselves.
Ellipsometry is a different technique that has been used for nondestructively measuring the thicknesses of films, given the complex index of refraction for the film and bare substrate. In ellipsometry, light of a known polarization and incident angle is reflected from the substrate, and relative shifts in the polarization state are measured. In a typical configuration the incident beam is a monochromatic beam that is passed through an adjustable polarizer (e.g., a polarizer and a compensator). The incident beam has both P and S polarizations (parallel to and perpendicular to the plane of incidence, respectively). The reflected beam is passed through an analyzing polarizer, and on to a detector. The adjustable polarizer is adjusted until the light reaching the detector is nulled. The setting of the adjustable polarizer that nulls the detected light determines the field reflection coefficients.
Ellipsometry is useful for analyzing the average film thickness value over an area. Scattering within the reflected beam causes the light from the repeated pattern and from the variations to be intermingled, which achieves averaging. This is a disadvantage, however, if the purpose of the measurement is to detect the variations, as opposed to detecting the average. For spatial resolution over a broad area using ellipsometry, one solution has been to focus the beam on a small portion of the surface and to scan the beam over the surface, one portion at a time. For a large surface this process may involve the scanning of thousands of portions, and may take hours to complete.
Cohn, R. et al., "Dynamic Imaging Microellipsometry; Theory, System Design and Feasibility Demonstration", in Applied Optics/Vol. 27, No. 22, November, 1988 and Cohn, R. and Wagner, J., "Mapping of Surface Film Parameters with Dynamic Imaging Microellipsometry", Quantitative Nondestructive Evaluation, Vol. 8B, discuss imaging ellipsometry devices. In each case, the reflected beam has an angle with respect to the surface normal, and part of the image is out of focus.